1. Field of the Invention
The present invention relates to an orthogonal frequency division multiplexing (OFDM) system and an OFDM-based cellular system, and more particularly, a method of virtually applying a multi-antenna technique to an OFDM system and an OFDM-based cellular system in order to eliminate an interference signal or obtain the effects of a spatial division multiple access (SDMA) technique.
2. Description of the Related Art
Orthogonal frequency division multiplexing (OFDM) offers a number of advantages. First, OFDM can remove inter-symbol interference (ISI) by inserting a cyclic prefix (CP) that is longer than an channel impulse response length into a guard interval between adjacent OFDM symbols. In addition, a receiving end can compensate for fading distortion using a single tap equalizer. In OFDM, modulation and demodulation can also be performed at high speeds using an inverse fast fourier transform (IFFT) and a fast fourier transform (FFT).
Such advantages of OFDM have resulted in the development of OFDM wireless communication systems. In other words, high-speed data transmission systems, such as digital audio broadcasting (DAB), digital video broadcasting (DVB), digital terrestrial television broadcasting (DTTB), local area network (LAN), and IEEE 802.16 broadband wireless access are being developed. These OFDM wireless communication systems are considered as core technologies for next-generation mobile communication and, accordingly, are actively being researched.
At the same time, a multi-antenna technique is also actively being researched. The multi-antenna technique is a transmission method that can provide high-speed data transmission in a reliable manner and increase system capacity. In the multi-antenna technique, a receiving/transmitting end uses a plurality of antennas. Some examples of the multi-antenna technique include a spatial division multiple access (SDMA) technique and a smart antenna technique. The SDMA technique, which is a multi-antenna technique, enables a plurality of users to simultaneously use a channel of the same frequency in the same cell in a cellular system. The smart antenna technique forms a beam in a desired direction using the arrangement structure of the antennas, thereby effectively removing the interference and increasing signal reliability. However, the multi-antenna technique can be applied only when the number of transmitting/receiving antennas increases, which, in turn, aggravates hardware complexity.
Another technique for removing interference is a multi-carrier code division technique. In the multi-carrier code division technique, a transmitting/receiving end with a single antenna removes the interference of the plurality of users or adjacent cells using an orthogonal code. One of the disadvantages of the multi-carrier code division technique is that the orthogonality can be disrupted according to the characteristics of a channel carrying an orthogonal signal and synchronization errors.